1. Filed of the Invention
The present invention relates to an eye movement analyzing device which observes, records and analyzes eye movement in any direction by focusing on the eye, particularly the iris, which moves following a target.
2. Prior Art
As a conventional eye movement recording method, EOG (electro-oculography) is well known and has been used clinically. According to this method, electrodes are placed on the skin around the eyes, and the inherent potential difference between the cornea and retina is used so that the potential differences produced as a result of eye movement are recorded. It is believed that the potential difference is nearly proportional to the rotating anqle of the eyes.
Another method conventionally used is PENG. This method uses the difference in reflection characteristics of infrared light irradiated on the cornea and sclera to detect eye movement.
There is another device for which the applicant of the present application has previously applied for a patent in Japan under the title "Eye movement Testing Device" (Japanese Serial No. 63-145425). This device uses invisible infrared light inside goggles. The goggles have a head-mount so that they can be placed on the head to create a "darkroom" in front of the examinee's eyes. The device thus designed outputs picture signals of eye movement through an infrared detecting television camera.
Among the prior art devices described above, because the EOG method places electrodes on the skin around the eyes, when electric potential changes on the skin are as frequent as those associated with eye movement, noise is generated and the potential difference between the cornea and retina becomes difficult to detect and thus is unstable. Closing the examinee's eyelids to remove the visual fixation that diminishes nystagmic reaction unfortunately causes the problem of vertical nystagmic reaction.
According to the method using reflected infrared light irradiated on the cornea and sclera, horizontal changes in eye movement can be precisely detected. However, the presence of the eyelids restricts eye movement, and the range of eye movement in the vertical and rotational directions becomes very narrow.
In comparison to these methods, the device of Japanese Serial No. 63-145425 uses goggles with a head-mount together with a darkroom in front of the examinee's eyes, facilitating mountinq and dismounting and enabling placement of a target inside the goggles. Thus, the need for elaborate equipment such as installation of separate targets, etc. is eliminated. Moreover, since the goggles are worn on the head, the examinee has a great deal of freedom of movement. The device directly observes, records, and analyzes eye movement with an infrared detector and offers various advantages over the prior art.
However, this device has problems. Though the movement or adjustment of the eyepiece at the specific position where the television camera is placed in front of the eye enables focusing on the proximate area of the iris, the curvature of the eye hinders simultaneous focusing on the center and peripheral areas of the eye. When the iris moves (together with eye movement), object distances change and it becomes difficult to obtain a constant, clear image of the eye, particularly the image around the iris.
A deep focal depth lens with the camera aperture significantly closed would be unable both the center and peripheral areas of the eye to be focussed on at the same time. However, strong illumination is required in this case which would affect the eyes.